Radioactivity

RADIOACTIVITY When Henri Becquerel was busy in performing experiments with uranium compounds, he accidentally placed the crystal of potassium uranyl sulphate (K2UO2(SO4)2.2H2O) over photographic plate wrapped with black paper. After developing the photographic plate he found the shadow of the crystal over it. He did the experiments several times and found that this crystal of potassium uranyl has emitted some mysterious rays that can penetrate the black paper. Later on, it was found that thorium compounds also emitted the same type of radiations.

Radioactivity: It is defined as the phenomenon of spontaneous emission of the radiations by an element or its compound. The substance which exhibits this phenomenon is called radioactive substance. Madam Curie and her husband Pierre Curie, the very famous names in the field of radioactivity. They discovered radioactive elements, polonium, thorium, radium and actinium. Some of these have high radioactive power than uranium. NATURAL AND ARTIFICIAL RADIOACTIVITY Radioactivity can be classified into two categories a) Natural radioactivity: If the substance emits radiations by itself, then it is called natural radioactivity. b) Artificial radioactivity or induced radioactivity: If the substance does not possess radioactivity but starts emitting radiation on exposure to rays from a naturally radioactive substance, then it is called induced radioactivity. NATURE OF RADIATION FROM RADIOACTIVE SUBSTANCE In 1904, Ernest Rutherford investigated the nature of the radiations emitted by the radioactive substance. He placed a piece of uranium in a lead box. He observed that when these radiations were subjected to electric or magnetic field these separated into three types called alpha (α), beta (β) and gamma (γ) rays. a) The rays which deflected slightly towards –ve electrode were named as Alpha (α) rays b) The rays, which deflected towards +ve electrode were called beta (β) rays. c) The rays, which remained undeflected, were the gamma (γ) rays.

Alpha (α) Rays: Nature: These rays are positively charged because these are deflected towards the –ve terminal of the electrode. It was also found that each α- particle carries two-unit +ve charge and has a mass nearly equal to 4 times that of hydrogen atom. These rays are represented by a symbol 24He because these rays consist of helium atoms that have lost 2 electrons. Velocity: The velocity of α- rays is found to be nearly 10% or 1/10 that of light. In fact, the actual velocity of α- particles depends upon the nature of substance from which they are ejected. Penetrating power: They don’t possess high penetrating power because of their large mass. These α -rays can be stopped by an aluminum foil of 0.1 mm thickness.

Ionising power: α- particles can ionize the gas through which they pass. This is because of the fact that they knock out the electrons from neutral gas molecules, when the highly energetic heavy α - particles strike these molecules. Effect on photographic plate and zinc sulphide screen: These rays affect the photographic plate just as ordinary light .Due to high kinetic energy, α - particles produce luminosity when they strike zinc sulphide screen. This phenomenon of producing luminosity is called fluorescence. Beta (β) Rays: Nature: These rays are negatively charged because these are deflected towards the +ve terminal of the electrode. It was also found that each β - particle carries the same charge and mass equal to that of electrons atom. These rays are represented by a symbol -10e or -10β. These are made of stream of electrons. Velocity: The velocity of β- rays is found to be nearly equal to that of light and faster than α - particles. As in case of α- rays, velocity of β - particles also depend upon the nature of substance from which they are emitted. Penetrating power: They possess high penetrating power than α-rays because of their smaller mass and high speed. These β -rays can be stopped only by an aluminum foil of 10 mm thickness. Ionising power: β- particles have poor ionizing power in spite of their high speed. This is because of the fact that they possess low kinetic energy due to their extremely small mass. Effect on photographic plate and zinc sulphide screen: Like α - particles, β -rays also affect the photographic plate but the effect in this case is much more. On the other hand β-rays do not have any significant effect on the zinc sulphide screen because of lower kinetic energy.

Gamma Rays (γ): Nature: These rays do not consist of material particles but they are electromagnetic waves having wave length of the order of 10-8 to 10-11 cm. They are electrically neutral because these are deflected by any electric or magnetic field. These rays are represented by a symbol γ.These are made of stream of electrons. Velocity: The velocity of γ - rays is found to be equal to that of light i.e.3x108ms-1. Penetrating power: They possess very high penetrating power than α-rays and β - rays because of their negligible mass and high speed. These γ -rays can penetrate through a sheet of an aluminum foil of thickness100 cm. Ionizing power: γ- particles have very poor ionizing power in spite of their high speed.. This is because of their non-material nature and negligible mass. Effect on photographic plate and zinc sulphide screen: These rays produce very little effect on the photographic plate and zinc sulphide plate.

Differences between the alpha, beta and gamma rays in tabulated form

CAUSE OF RADIOACTIVITY: With the help of n/p i.e. neutron proton ratio we can find the stability of the nucleus. From the graph plotted between neutrons (n) and number of protons. It is concluded from the graph that

i) ii)

Light nuclei (A<20) have n/p or (A-Z)/Z ratio close to unity. For heavy nuclei (A>20) the n/p ratio increases progressively due to dominance of the number of neutrons. Non- radioactive nuclei are more stable because n/p ratio is between116 as clear from the graph. These nuclei lie in the dotted region of the plot, which is also called stability belt or stability zone. The nuclei of the elements which are not in the stability belt/zone are unstable.The n/p ratio of these unstable nuclei is either less than 1 or greater than 1.6. These nuclei will disintegrate and give out α, β, γ – rays to attain stability. The process of disintegration continues till n/p ratio falls with in the stability range.

EXAMPLES 1) n/p ratio decreases, by the emission of β –particles γ

(iii) n/p ratio increases either by the emission of α – particles or by the emission of positron or capture of orbital electron (K-electron capture)

(iv)

Capture of K-electron:

The process of K capture occurs only in those nuclei which have low n/p ratio and insufficient energy for the emission of positron. In this case nucleus captures an electron from K shell and converts the proton into neutron, which remains in the nucleus (p++ e- → n). This decreases the atomic number by one unit. As a result vacancy is created in the shell, which

is filled by the electron from the higher level and producing X –rays or release of neutrino. This is called K- capture.

Positron: Anderson discovered it. It has the same mass or same charge as on the electron but opposite with sign. Denoted as +10e. A positron is formed in the conversion of a proton into neutron.

Positrons are unstable and quickly combine with electrons producing γ -rays.

Meson: It has a mass in between that of electron and proton. Its mass is about 273 times more than that of electron can have positive or negative charge or may be neutral in some cases(denoted as ∏+ , ∏- or ∏0)

Neutrino: These are mass less and charge less particles which are emitted during the emission of β -particles and positrons respectively.

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